Move StreamTransformerBase to core package

Author: reakaleek

src/api/Elastic.Documentation.Api.Infrastructure/Adapters/AskAi/StreamTransformerBase.cs renamed to src/api/Elastic.Documentation.Api.Core/AskAi/StreamTransformerBase.cs

@@ -8,11 +8,9 @@
 using System.Runtime.CompilerServices;
 using System.Text;
 using System.Text.Json;
-using Elastic.Documentation.Api.Core;
-using Elastic.Documentation.Api.Core.AskAi;
 using Microsoft.Extensions.Logging;
 
-namespace Elastic.Documentation.Api.Infrastructure.Adapters.AskAi;
+namespace Elastic.Documentation.Api.Core.AskAi;
 
 /// <summary>
 /// Represents a parsed Server-Sent Event (SSE)